Link aggregation, or LAG (link aggregation group) is a computer networking term to describe various methods of combining, that is, aggregating or grouping, multiple physical network links in parallel to increase throughput beyond what a single link could sustain, and to provide redundancy in case one of the parallel links fails. LAG allows bonding two or more physical links into a logical link, for example, between two switches, or between a server and a switch such that a Media Access Control (MAC) client can treat the LAG as if it were a single link.
Link aggregation, or a link aggregation group (LAG), may also be referred to as port trunking, link bundling, Ethernet/network/Network Interface Controller (NIC) bonding, or NIC teaming, and encompasses vendor-independent standards such as the Link Aggregation Control Protocol (LACP) for Ethernet defined in IEEE 802.1ax or IEEE 802.3ad, as well as various proprietary solutions.
Multiple Link Aggregation Group, also referred to as Multi-Switch LAG, or as Multi-Chassis LAG, and hereinafter referred to simply as MLAG, adds node-level redundancy to the normal link-level redundancy that a LAG provides. This allows two or more nodes to share a common LAG endpoint. The multiple nodes at the common endpoint present a single logical LAG to the remote end of the aggregated link. Currently, MLAG is vendor-specific, and is not covered by a standard, such as an IEEE standard.
The assignee of the present invention, Extreme Networks, Inc., offers a proprietary layer-2 ring resiliency protocol known as Ethernet Automatic Protection Switching (EAPS) that provides for loop-free operation and very fast ring recovery in the event of a link failure. Other vendors have developed similar protocols, such as Ethernet Protection Switching Ring (EPSR), Rapid Ring Protection Protocol (RRPP), ZTE Ethernet Smart Ring (ZESR). Further, Ethernet Ring Protection Switching, or ERPS, is an effort at ITU-T under the G.8032 Recommendation to provide similar protection and recovery switching for Ethernet traffic in a ring topology and at the same time ensure that there are no loops formed at the Ethernet layer. These protocols are referred to hereafter as fault tolerant ring topology protocols, or simply, ring resiliency protocols. When a ring resiliency protocol, such as EAPS, and MLAG are deployed together in a network, some of the benefits of MLAG are not realized, such as:                1.) The MLAG Inter Switch Connection (ISC) links are generally not used in steady state operation (that is, when no failures occur in the ring) for unicast traffic as MLAG favors local switching. However, when a ring resiliency protocol, such as EAPS, is also configured and in use, the ISC link ports are also the ring resiliency protocol (EAPS) ring ports which can defeat this MLAG benefit; and        2.) The number of hops through the network, and the associated latency increase, can double due to having 2 MLAG peer switches per location in a resilient ring, that is, in a ring in which a ring resiliency protocol is operating, such as an EAPS ring.        
FIG. 1 illustrates an exemplary network topology 100 in which both link aggregation, e.g., MLAG, and ring resiliency, e.g., EAPS, protocols are deployed. The topology comprises four sites 110, 120, 130 and 140. Each site has multiple nodes that can be sources or targets for data packet flows. For example, site 120 includes nodes 121 and 122 that can send and receive data packet flows to or from the network. Nodes 121 and 122 could be end user nodes or edge switches in the network. FIG. 1 illustrates a single packet flow starting at source node 122 at site 120 and traversing from site 120 to site 140, in network 100. In the example topology, ring node 146 is the ring resiliency protocol (e.g., EAPS) master node, while all other ring nodes 115, 116, 125, 126, 135, 136, and 145, are ring resiliency protocol transit nodes. Aggregated link 156 is blocked as depicted at 170 by the ring resiliency protocol. Thus, as illustrated by arrows 131-139, packets received by source node 122 at site 120 that hash to switch 120 and that are destined to site 140 must traverse 7 ring nodes 125, 126, 115, 116, 135, 136 and 145, including traversing 3 inter-switch communication (ISC) links 160, 162, and 164, as well as aggregated links 150, 152 and 154. This generally requires extra provisioning and, thus, cost in the network.